Immune responses were examined in cattle between 3-5 years after experimental inoculation with bovine immunodeficiency virus (BIG) and/or bovine leukemia virus (BLV). Lymphocyte proliferative responses to Con A or to allogeneic lymphocytes with foreign major histocompatibility complex molecules (allo MHC) were determined by 3H-thymidine incorporation assays. Antigen-specific antibody and lymphocyte proliferative responses were measured following vaccination with tetanus toxoid (TT) and bovine herpes virus-1 (BHV-1). Lymphocytes from BIV-infected cattle had significantly (p<0.05) reduced proliferative responses to Con A, but responses to allo-MHC and TT did not differ from those of uninfected controls. BIV infection also had little effect on TT-specific antibody responses in vivo. In contrast, BLV-infected cattle had significantly increased secondary antibody responses to vaccination with TT, as well as enhancement of antibody responses to BHV-1. Co-infection with BIV did not alter the BLV effect, suggesting a lack of significant interaction between the two viruses in vivo. Numbers of circulating mononuclear cells were also higher in BLV-infected cattle, which was attributable to increases in both T and B cell numbers. Unstimulated lymphocytes from BLV-infected cattle had significantly increased spontaneous uptake of 3H-thymidine in vitro. When differences in counts per minute were analyzed, lymphocytes from BLV-infected cattle had slightly increased proliferative responses to Con A, but no consistent alternations in responsiveness to allo-MHC, TT, or BHV-1. The observed increase in antibody responses to non-BLV antigens suggests that at least in clinically asymptomatic cattle, BLV infection may cause a non-specific B cell activation.